1. Field of the Invention
This invention relates to a device for controlling an electromagnetic clutch on a vehicle electromagnetic clutch control device which performs a clutch transmission torque control operation by controlling the supply of current.
2. Prior Art
A conventional vehicle electromagnetic clutch control device has been disclosed, for instance, by Japanese Patent Application (OPI) No. 31533/1987. The control device is as shown in FIG. 3.
In FIG. 3, reference numeral 100 designates a micro-computer. The micro-computer 100 obtains a current instruction signal according to travel control data SD and engine control data SE, and applies it to a digital-to-analog (D/A) converter 111.
In response to the current instruction signal, the D/A converter 111 outputs an analog signal, namely, a current instruction signal IS, which is applied to the positive input terminal (+) of a pulse width modulation (PWM) control comparator 112, to the negative input terminal (-) of which an output current feedback signal IF provided by a current detecting amplifier 7 is applied.
The PWM control comparator 112 obtains the difference between the current instruction signal IS and the output current feedback signal IF, and performs a pulse width modulation according to the difference. The output of the comparator 112 is applied through a resistor 205 to the base of a transistor 203.
The base of the transistor 203 is grounded through a resistor 204, the emitter is grounded directly, and the collector is connected to the base of a PWM transistor 2 and further connected through a resistor 201 to a power source (voltage V.sub.IG).
The emitter of the PWM transistor 2 is connected to the power source, and the collector is connected to an output terminal 113 and grounded through a circulation diode 3. The output terminal 113 is connected through an electromagnetic clutch 6 to another output terminal 114. The electromagnetic clutch 6 comprises slip springs 601 and 602, and a clutch coil 603. Clutch current is supplied through the slip springs 601 and 602 to the clutch coil 603.
The microcomputer 100 is adapted to output a clutch release signal SO, which is applied through a resistor 408 to the base of a transistor 406. The base of the transistor 406 is grounded through a resistor 407, the collector is connected through resistors 404 and 405 to the power source, and the emitter is grounded.
The connecting point of the resistors 404 and 405 is connected to the base of a transistor 403. The emitter of the transistor 403 is connected to the power source, and the collector is connected through a resistor 402 to the base of a quick-break transistor 4.
The collector of the quick-break transistor 4 is connected to the output terminal 114, and connected through a constant voltage diode 401 to the base thereof. The emitter of the quick-break transistor 4 is grounded through a current detecting resistor 5. The latter 5 is to detect a clutch current.
Both terminals of the current detecting resistor 5 are connected through a resistor (reference resistor) 701 and a resistor (adjust resistor) 702 to the positive input terminal (+) and negative input terminal (-) of the current detecting amplifier 7, respectively. The negative input terminal (-) of the current detecting amplifier 7 is connected through a resistor (feedback resistor) 703 to the output terminal thereof, at which the above-described output current feedback signal IF is provided.
The operation of the vehicle electromagnetic clutch control device thus organized will be described. A control unit (not shown) calculates a vehicle speed and an engine speed (a number of revolutions per minute of an engine). The aforementioned SD is applied to the microcomputer 100 according to the vehicle speed and the engine speed.
Thereafter, the microcomputer 100 receives the engine control data SE. The microcomputer 100 output a current instruction signal IS according to the travel control data SD and the engine control data SE.
The current instruction signal IS thus outputted is applied to the D/A converter 111, where it is converted into the analog signal IS, which is applied to the positive input terminal (+) of the PWM control comparator 112, to the negative input terminal (-) of which the output current feedback signal IF provided at the output terminal of the current detecting amplifier 7 is applied. Hence, the PWM control comparator 112 compares the current instruction signal IS with the output current feedback signal IF to output a difference signal.
The difference signal is subjected to pulse width modulation. The output of the PWM control comparator 112 is applied through the resistor 205 to the transistor 203 to turn the latter on and off, thereby to turn the PWM output transistor 2 on and off. That is, the PWM output transistor 2 is rendered conductive (on) or non-conductive (off) according to the pulse width with which the difference signal is pulse-width-modulated.
Thus, the application of a clutch current to the electromagnetic clutch is controlled according to the on and off states of the PWM output transistor 2. More specifically, when the PWM output transistor 2 is rendered conductive (on), the clutch current is allowed to flow in the electromagnetic clutch 6 through the output terminal 113, and when the transistor 2 is rendered non-conductive (off), the clutch current is not allowed to flow in the clutch 6. When the PWM output transistor 2 is non-conductive (off), a circulation current is not allowed to flow in the electromagnetic clutch 6.
On the other hand, the on-off operation of the quick-break transistor 4 is controlled by the release signal SO. When the release signal SO is applied through the resistor 408 to the transistor 406 to turn the latter 406 on, the transistor 403 is turned on, as a result of which the quick-break transistor 4 is also turned on. Hence, when the PWM output transistor 2 is rendered conductive (on) by the difference signal pulse-width-modulated, the clutch current is allowed to flow from the power source in the closed loop of the PWM output transistor 2, the output terminal 113, the electromagnetic clutch 6, the output terminal 114, the quick-break transistor 4, the current detecting resistor 5, and ground.
In this operation, the clutch current is detected by the current detecting resistor 5, and a voltage drop is developed across the latter 5. The voltages at both terminals of the current detecting resistor 5 are applied through the resistors 701 and 702 to the input terminals of the current detecting amplifier 7, respectively, so that the latter 7 outputs the above-described output current feedback signal IF.
As is apparent from the above description, the clutch current is allowed to flow in the electromagnetic clutch 6 according to the PWM modulation signal of the difference signal provided by the PWM control comparator 112, to drive the electromagnetic clutch. On the other hand, the electromagnetic clutch 6 is released by the clutch release signal SO outputted by the microcomputer 100.
The conventional vehicle electromagnetic clutch control device thus constructed suffers from the following difficulties: In the control device, the D/A converter 111 and the PWM control comparator 112 occupy relatively larger areas on the printed circuit board, thus making the control device bulky.
The current instruction signal outputted by the microcomputer 100 does not reflect variations in supply voltage. Therefore, it is impossible to correct the actual clutch current accurately with respect to the PWM modulation signal outputted by the PWM control comparator.